Apparatus for coating a continuous web

ABSTRACT

Two or more low coverage coatings are applied substantially simultaneously to a web travelling at high speed by first trowelling a very thin sub-coat onto the web with a trailing blade coater to prepare the web surface for an over-coating of the same or compatible coating material, and then flowing the over-coat down the top side of the trailing blade and off the trowelling end thereof directly onto the sub-coat. Two or more layers of fluid coating composition can be flowed down the top of the trailing blade and onto the trowelled sub-coat in distinct layer relationship with each other and the sub-coat.

United States Patent 11 1 1111 3,756,195 Mercier 1 Sept. 4, 1973 [54]APPARATUS FOR COATING A 3,200,323 9 1905 Miller ct a1. 1111 411 xCONTINUOUS WEB 3,348,526 10/1967 Neubauer 118/410 3,484,279 12/1969Clark et 118/413 X 1 Inventor: J s p l Rochester, 3,496,005 2 1970lshiwata et =11 113 411 x 73 Assi nee: Eastman Kodak Com an J g pPrimary ExaminerJohn P. McIntosh Rochester, NY. Atl0rneyKar1 T. Naramore[22] F11ed: Aug. 5, 1971 21 Appl. No.: 169,489 ABSTRACT Two or more lowcoverage coatings are applied sub- Related Applicanon Dam stantiallysimultaneously to a web travelling at high [62] of 55323 July 1970,speed by first trowelling a very thin sub-coat onto the web with atrailing blade coater to prepare the web surface for an over-coating ofthe same or compatible [:2] U.S. Cl. 118/126, 118/41 1 coating materialand then flowing the ovepcoat down Ilrt. Clthe p Side of the trailing ld d f the Dwelling [58] Field of Search 118/411, 412, 413, end thereofdirectly onto the subcoat Two or more 1 18/126 layers of fluid coatingcomposition can be flowed down the top of the trailing blade and ontothe trowelled sub- [56] References C'ted coat in distinct layerrelationship with each other and UNITED STATES PATENTS h sub oat2,695,005 11/1954 Lewin et a1. 118/412 2,941,898 6/1960 Wynn 1 18/412 X8 Chums 8 Drawing figures 3,070,066 12/1962 Faeberm. 118/413 BLADEDEFLECTION,

GOA TING A/VGL E PATENIED SE? 4 I975 SHEU 1 BF 5 BLADE DEFLECT/O/V C04TING PATENTEflszr 4 me:

sum 2 or 5 SHEET 3 [If 5 PATENTED SEP 4 I975 skrsslws PATENTEU 3H 4 I875SHEET 5 [IF 5 BLEND WITH LARGE RADIUS mag FIG. 7

GRAD/US RAD/U8 r02, BLADE THICKNESS APPARATUS FOR COATING A CONTINUOUSWEB This is a division of application Ser. No. 55,323, filed July 16,1970, now U.S. Pat. No. 3,627,564.

The present invention relates to coating a continuous web, andparticularly to apparatus for applying very low coverage with reasonableuniformity in thickness to a web travelling at high speed.

A very difficult coating problem is the application of very lowcoverages with reasonable thickness uniformity to a continuous webtravelling at high speed. One approach which is used in the commercialcoating field is the trailing blade coater. This device, however,operates as a smoothing coater, filling the low areas of the web surfaceand scraping clean the high points. The resulting coating is notsatisfactory for use in arts, e.g., the photographic art, where areasonable uniformity of thickness of coating is desired, although it isvaluable as a surface coater for the stock. One problem encountered withtrailing blade coaters is that the fluid coating composition has atendency to dry on the blade edge and eventually a build-up of solidcoating material is formed on the blade edge which produces longitudinalstreaks in the coating.

Slide coaters have been used in the art of coating photographic films orplates and wherein the emulsion flows down an inclined surface whichextends from a hopper to the surface to be coated, see U.S. Pat. No.401,771, U.S. Pat. No. 2,761,417 shows a multiple slide hopper by meansof which two or more layers of fluid coating composition can besimultaneously applied to a continuous web in distinct layerrelationship. While these slide coating techniques are capable ofapplying reasonably uniform coatings to a continuously moving web, theyare somewhat limited in coating speed because at high speed air tends tobecome entrained beneath the coating as it comes off the slide or iscoated from a head, as the case may be.

The primary object of the present invention is to provide coatingapparatus which combines the trailing blade and slide methods of coatingin such a way as to overcome the shortcomings of both and provide meansby which one or more coating compositions of very low coverages and ofreasonable thickness uniformity can be applied to a continuous web athigh speeds.

Another object is to provide apparatus applying very low coveragecoatings to a web travelling at high speed. The invention also comprisesmeans for first trowelling a very thin sub-coat onto the web surfacewith a trailing blade coater so as to prepare the surface for anovercoating of the same or compatible coating material, and means forthen flowing the over-coat down the top side of the trailing blade andoff the trowelling end thereof directly onto the sub-coat.

The invention further provides apparatus for coating as described aboveby the use of which two or more layers of fluid coating composition(s)can be flowed down the top surface of the trailing blade and be floweddi rectly onto the trowelled sub-coat in distinct layer relationshipwith each other and the sub-coat. I have chosen to designate the coatingapparatus of the invention trailing blade slide coater."

The novel features that I consider characteristic of my invention areset forth with particularity in the appending claims. The inventionitself, however, both as to its organization and its methods ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings in which:

FIG. 1 is a schematic view of a conventional trailing blade coater, andshowing the system parameters thereof;

FIG. 2 is a side view, partly in section, showing a two layer trailingbladeslide coater constructed in accordance with one embodiment of thepresent invention;

FIGS. 3 and 4 are views similar to FIG. 2, but showing other ways inwhich the undercoat or sub-coat may be applied to the web;

FIG. 5 is a view similar to FIGS. 2-4, but showing another embodiment ofthe apparatus adapted to apply three layers of coating to a web indistinct layer relationship; and

FIGS. 6, 7 and 8 are schematic views showing different blade edgegeometries on an enlarged scale.

Generally speaking, the present invention for applying a reasonablyuniformly thin layer of a given coating solution to the surface of a webmoving continuously at high speed comprises means for applying a fluidcomposition to the surface of the web, and including a flexiblestationary blade for trowelling said composition as a sub-coating on theweb surface by moving the web past the end of a trowelling surface ofsaid flexible stationary blade that is pressing against the web surfaceto fill any low areas in the web surface and thereby provide a smoothwet surface on the web; means for continuously metering a given quantityof said fluid composition, or another coating composition compatiblewith said sub-coat, in the form of a layer onto a nontrowelling surfaceof said trailing blade at a point spaced above the flexible end thereofso that said layer of coating composition, under the influence ofgravity may flow down said trailing blade to form a uniformly thin layerwhich ultimately flows off the end of the blade and onto the surface ofthe sub-coat. With apparatus of the present invention two or more layersof fluid composition may be simultaneously applied to the sub-coat indistinct layer relationship with each other and the sub-coat if desired.The blade applied sub-coat acts asa seal to prevent air entrainmentunder the subcoat( s) which is a major problem in slide or bead coaterswhen operating at high speeds. The moving liquid layer(s) flowing offthe trailing edge of the blade also acts to improve the blade operationby preventing drying of coating material on the blade. edge.

Referring to FIG. '1, a conventional blade coater is schematically shownto clarify the system parameters of such a coater as they supply to thepresent invention as will be described below. The web, W, to be coatedis supported in a smooth condition on the surface of a coating roll 10moving in a counterclockwise direction as shown by arrow 11. The web tobe coated may be raw paper, pre-coated paper, plastic film base, etc.Riding on the exposed surface of the web W as it moves around saidcoating roll is the flexible end 12 of a trailing blade 13 whose otherend 13" is clamped, or otherwise fastened, to a rigid support 114. Thetrailing blade may be made of any suitable springy material, e.g.stainless steel, blue steel, etc., but it should be'made of a materialwhich: will not be readily corroded by the fluid composition to be usedand should be of such hardness that the-trailing edge will not be undulyworn away by the rubbing contact it has with the web. The surface of thecoating roll 10 may be metal or it may have a resilient surface which isengaged by the web.

In conventional operation of such a trailing blade coater, an excess ofa fluid coating composition is applied to the web surface ahead of theblade and a puddle of the composition builds up ahead of, and adjacentto, the blade across its entire width. The flexible end of the bladethen trowels the composition which passes beneath the trailing edgethereof and acts as a smoothing coater, filling in the low areas of theweb and scraping clean the high points. The trowelling action of thetrailing blade can be adjusted by varying the blade cantilever, thestiffness of the blade, and the coating angle of the blade, eachindividually or any two or three of these parameters in combination. Inother words, it will be appreciated that the more pressure that thetrailing edge of the blade exerts upon the surface of the web the lesscoating composition will flow beneath it. As will appear later, theamount of coating composition which will flow past the blade will alsodepend upon the physical characteristics of the coating composition. Itwill be appreciated that unless a perfectly flat web is moving past thetrailing blade the thickness of the coating trowelled onto the web willnot be uniformly thick. In other words, a trailing blade coater isprimarily useful only in filling in the lower areas of a web surface andconditioning the web surface so that it is smooth.

Looking now at FIG. 2, a trailing blade slide coater for applying twolayers of coating solution to the surface of a web, and constructed inaccordance with one embodiment of the present invention, is shown andwill be described. The object here is to obtain a layer of a fluidcoating composition on the web which is thin and uniformly thick. To dothis, a sub-coat is first applied to the web surface by a trailing bladecoater to provide a smooth wet surface on the web onto which the finalcoating of desired uniform thickness is applied and/or to condition thesurface of the web to receive and hold the final over-coat. Thissub-coat can be the same composition as the over-coat, or it can be adifferent composition. If it is a different coating composition, thenthe only requirement is that it be compatible with the over-coatcomposition so that the over-coat layer(s) will readily spread andadhere to the sub-coat and not be adversely affected by the sub-coatsubsequent to their application.

As in the previously described trailing blade coater, the web W is heldin wrapped relation with a coating roller to keep it smooth as itapproaches and passes the coating point. A trailing blade 13' is clampedbetween two sections 16 and 17 of a coating hopper held together bysuitable means, e.g. bolts (not shown). The hopper section 17 isprovided with a cavity 18, one wall of which is defined by theclamped-end 13'' of the trailing blade. The fluid sub-coat compositionSc is fed into the cavity 18 through an inlet 19 by a pump P. Thesubcoat composition exits in a layer L through a discharge slot 20, theupper side of which is defined by the lower side of the trailing blade.As a result, the layer L of the sub-coat composition flows down theunder side of the trailing blade to form a puddle or bead 21 extendingcompletely across the deflected end 12' of the blade and between it andthe web. As the web moves across this puddle or bead 21, it tends tocarry some of the sub-coat with it, and this is trowelled over the websurface by the the trowelling surface at the deflected end of the blade.This trowelling action of the blade provides a smooth wet surface on theweb since the blade causes the sub-coat to fill in the low areas of theweb and scrapes clean the high points.

The hopper section 16 has a cavity 22 formed therein which is covered bya third hopper section 23 fastened to hopper section 16 in any suitablemanner, eg by bolts (not shown). Leading from the cavity 22 there is adownwardly inclined, elongated discharge slot 24 formed between spacedfaces of hopper sections 16 and 23. The discharge slot 24 will be thesame length as the width of the web to be coated and its exit isdirectly obliquely onto the uppermost surface of the blade 13' at apoint spaced above its deflected end 12'. The fluid coating compositionOc which is to be coated onto the web in a thin layer of uniformthickness is fed 'into the cavity 22 through an inlet 22' by a pump P ofthe constant discharge type. As the over-coat composition 00 is fed intothe cavity at a constant rate it will exit from the discharge slot 24 inthe form of a ribbon or layer onto the uppermost or non-trowellingsurface of the downwardly inclined trailing blade down which it flows bygravity to form a layer L This layer L as it flows down the uppermostsurface of the blade becomes uniformly thin before it slides off the endof the blade and onto the surface of the sub-coat which has beentrowelled on the web surface by the blade. The deflected end 12 of thetrailing blade intersects the web surface at an obtuse angle so that thelayer L, of over-coat flowing off the end thereof will flow onto thelayer of sub-coat in such a way as to maintain a distinct layerrelationship between the two. This distinct layer relationship betweenthe two coatings is not important if the two coating compositions are ofthe same material, but it is important in certain cases where thecoating compositions are different and a distinct layer relationshipbetween them is desired, e.g. in certain photographic color materialswhere the several color sensitive and filter layers must be applied indistinct layer relationship.

The puddle 21 of the sub-coat composition formed between the web surfaceand the under side of the trailing blade will prevent the entrainment ofair beneath the over-coat layer which slides off the end of the bladeand onto the sub-coat. At the same time, the trowelling action of theblade on the sub-coat composition will further prevent air entrainmentunder the sub-coat. It will thus be seen that this method of coatingovercomes the air entrainment problem which is most prevalent in highspeed coating procedures. Also, since there is a layer L of over-coatcomposition continuously flowing off the trailing end of the blade, thetrailing blade operation is improved by the prevention of drying ofcoating material on the blade edge.

The method used in applying the sub-coat solution was found to have amajor influence on the coating procedure. The method of application bestsuited for a particular situation depends mainly upon the properties ofthe sub-coat composition to be applied, as will be more fully set forthbelow.

FIG. 3 shows a two-layer coating system similar to that shown in FIG. 2but having a different type of subcoat applicator. In this embodimentthe sub-coat composition is applied to the web surface ahead of thetrailing blade 13 from a manifold applicator 30 extending transverselyof the web and having one, or more, inlets 31 into which the fluidcoating composition is fed by a pump, not shown. The manifold applicatoris provided with a plurality of discharge openings 32 spaced along itslength through which the coating material issues in the form ofindividual streams spaced across the'width of the web surface. Thesestreams of coating material deposited on the web build up into a puddle21 behind the trailing blade, said serving to distribute the coatingmaterial uniformly across the said puddle immediately ahead of theblade.

FIG. 4 shows another embodiment of a two layer system similar to thatshown in FIGS. 2 and 3 but having a different type of subcoatapplicator. In this embodiment the sub-coat is supplied from a reservoirtype applicator 35 wherein a relatively large quantity of coatingmaterial is maintained on the web surface immediately behind thedeflected end 12' of the trailing blade 13'. The coating material is fedinto this reservoir through an inlet 36 by means of a pump not shown.

The present method of coating a web can be used to simultaneously applya plurality of different fluid coating compositions in distinct layerrelationship at high speeds. In FIG. 5 an embodiment is shown which issuitable for applying three different layers of coating composition ontoa web surface at the same time. Here the web W to be coated is passedaround a coating roll in a counter-clockwise direction and its surfaceis engaged by the deflected end 12' of trailing blade 13' clampedbetween two sections 16' and'17' of a hopper. The fluid coatingcomposition Sc which is to form the sub-coat is fed onto the web surfaceahead of the trailing blade 13' from a plurality of tubes 40, only oneof which is shown, spaced across the web. The sub-coat material is fedinto these tubes by a pump, not shown, and the streams of coatingreaching the back side of the deflected end of the trailing blade mergeinto a puddle 21' from which the material flows under the blade and istrowelled into a layer L on the web surface thereby.

As in the FIG. 2 embodiment the fluid coating composition Sm which is toform the middle coat is pumped into a cavity 22 in the hopper section16' by a constant discharge pump and issues therefrom through adischarge slot 24 in the form of a layer L which is directed onto theuppermost surface of the trailing blade. The fluid coating compositionSt which is to form the top coat is fed into a cavity 42 in anotherhopper section 43 by a pump of the constant discharge type. This coatingcomposition issues in the form of alayer L from a discharge slot 45defined by a surface 46 on the hopper section 43spaced from the top ofhopper section 23. This layer flows down the inclined surface 47 ofhopper section 23' and onto the top of the layer L of the middle coatissuing from cavity 22'. The two layers L, and L, then flow down theuppermost surface of the trailing blade in distinct layer relation andflow off the end of the blade onto the layer L of sub-coat while stillmaintaining a distinct layer relationship with each other and thesub-coat. The final thickness of the individual layers L and L, aredetermined, not by the width of the discharge slots through which theyissue from and supply cavities 22' and 42, respectively, but by the rateat which they are pumped into their respective cavities. The coveragethickness of the sub-coat will depend upon the smoothness of the websurface, the physical properties of the coating composition used and thepressure exerted by the trailing blade in trowelling the coating.

The blade edge geometry has a major influence on the ability of thetrailing blade slide coater process to effectively coat a continuousweb. FIGS. 6, 7 and 8 show three types of edge .geometries used intrials of this method and apparatus of coating. The blade edge shown inFIG. 8 was found to be best suited for use with the trailing blade slidecoater process.

The edge shown in FIG. 6 is well suited for applying over-coat layerssince the sharp edge makes it easy for the over-coat-composition(s) toslide onto the web surface. A disadvantage of this type of blade edge isits high sensitivity to any paper fibers (if the web is a raw paperstock) or foreign particles that may be present on the web surface. Thefibers and foreign particles are retained at the blade edge and causestreaks in the coating. The sharp edge blade tends to wear quicklycausing additional problems.

The blade edge as shown in FIG. 7 is well suited to applying a bladecoat to acontinuous web. The rounded edge will apply a coating that hasa good finish, and it is not largely influenced by dirt and paperfibers. A disadvantage with this blade is that it becomes extremelydifficult to slide an over-coat off the blade and onto a continuous web.

The blade as shown in FIG. 8 is a compromise between those shown inFIGS. 6 and 7. It is designed to utilize the advantages of both theknife edge blade and the rounded edge blade. In a series of coatingexperiments performed with the trailing; blade slide coater, it wasfound that the coating and trailing blade angles (see FIG. 1) had nomajor influence on the coating technique. Both angles were variedthrough large ranges.

The trailing blade slide coating technique has been evaluated on acommercial coating machine using aqueous solutions of gelatin, methanolsolutions of polyvinyl acetate (hereinafter referred to as PVA), aqueoussolutions of carboxymethyl cellulose (hereinafter referred to as CMC)and aqueous dispersions of baryta and aqueous dispersions of clay-caseinas the blade coated materials. Aqueous solution of gelatin, PVA and CMCwere also used for the slide coat. Satisfactory coverages ranged from0.06 to 1.40 lbs/ ft for the slide coat and 0.008 to 0.50 lbs/100 ft forthe blade coat. Coating speeds ranged from 200 to 1,030 ft/rnin, thishigh speed being the upper limit which the machine used was capable of.In some cases dyes were mixed into the coating fluids so that variouslayers of a coating sample could be differentiated. For the majority ofthese trials a raw paper stockwas used as the continuous web. Papercoated with polyethylene or baryta were also tried in order to determinethe effect of precoated paper webs on this coating technique.

The following tables list the experimental details of different trialswhich were made to evaluate possible limitations of the present coatingtechnique. Table 1 lists trials in which only the blade coat, orsub-coat, was applied to the continuous web. Table 2 lists trials inwhich the blade coat and the overcoat (slide coat) were appliedsimultaneously. Triple coats applied in a like manner are listed inTable 3. Table 4 gives the viscosities for the fluid compositions used.

TABLE 1 SAMPLES OF SUB-COAT TRIALS ONLY Wet Machine Trial MaterialSolids Coverage Speed lbll00 Ft ft/min l Gelatin 5 0.15 950 2 Gelatin 150.32 200 3 Gelatin 15 0.35 200 4 Gelatin 15 0.12 850 Gelatin is 0.50 950DISCUSSION OF TRIALS 6 Gelatin I5 0.35 300 7 Gelatin 20 0.14 300 l.Sub-coat (blade coat) Fluid Compositions. See FIG. 8 Gelatin 25 0.48 6201 T ble 1 9 Gelatin 30 0. I a 260 10 CMC 3 016 300 A. Aqueous GelatinSolutions 1; $3 5 3 8-}: 3:8 5 In contrast to elastic solids, mostliquids are consid- 13 pv 310 ered to possess no elasticity or rigiditywhen sheared barytfl 64 310 and should respond immediately to anyattempt made IS baryta 64 0.11 310 t d f th Th h th 16 clawascin 45 mo 0e orm em. ere are, owever, IqLll s at gener- 17 clay-casein 57 0.14 300ate measurable normal stresses when sheared and show Polycoated paperstock 10 l l Bawwcoated paper stock both viscous and elastic effects.These llqUIdS are called All other trials raw paper stock. viscoelasticsolutions or dispersions.

Solids, coverage, Overcoat Solids, coverage, speed, percent Ib./l00ft.material percent lb./I00It. It./min.

I5 0. 20 5 1. 00 200 I5 0. 16 5 0. 29 800 0. 35 5 0. 11 500 15 0, 50 50. 11 500 15 0. l2 5 0. 2'.l 850 l5 0. 35 5 0. 30 200 ll. 11 5 0. 50 300.20 0. l1 5 0. 20 300 20 O. 20 5 0. 17 920 0. 5 0. 06 1030 25 0. 48 5 0.18 620 3 0. 17 5 0. 06 300 3 0. 11 5 0. 12 990 3 0. 23 5 0. 09 950 3 0.23 5 9. 15 950 3 0. 23 5 0. 22 950 3 0. 09 20 0. 54 940 3 0. 14 20 0. 44670 3 0. 20 12 0. 30 370 3 0. 16 18 0. 09 300 3 0. 16 18 0. 16 300 3 0.16 18 0. 23 950 3 0. 16 18 0. 10 950 8 0. 09 5 0. 20 310 8 0. 12 5 0. 06980 8 0. 12 5 0. 10 980 8 0. 12 5 0. 16 980 8 6. 12 3 0. 06 970 8 0. 123 0. 15 970 8 O. 12 3 0. 970 8 0. 09 3 0. 19 360 8 0. 09 3 0. 34 360 80. 10 3 0. 17 650 8 0. 18 3 0. 26 810 12 0. 15 3 0. 80 360 20 0. 15 3 0.60 360 20 0. 08 18 0. 08 950 64 0. 14 5 0. 19 310 64 0. 28 5 0. 11 95064 0. 30 5 0. 11 950 64 0. 37 5 0. 06 950 0. l0 5 0. 310 45 0. 10 5 0.60 310 45 0. 14 5 0. 13 930 I Baryta-coated paper stock. 1 Polycoetedpaper stock.

Nora-A11 other trials raw paper stock.

TABLE 3.SAMPLES WITH SUB-COAT, MIDDLECOAT, AND OVERCOAT Coverage,Coverage, Coverage, Machine Sub-coat Solids, lb./100 Middlecoat Solids,lb./100 Overcoat Solids lb./100 speed, Trial material percent [tflmaterial percent It. material percent It (t./mln.

I Gelatin 20 0.15 Gelatin. 5 5 0.70 300 OMS"--. 3 0.10 ..do 5 5 0.54 300Clay casein 57 0. 14 .do 18 18 0. 54 300 Norm-All trials on raw paperstock.

TABLE 4 Windle and Beazley, The Role of Viscoelasticity in 55 BladeCoating," Tappi, August 1968, Vol. 5, No. 8, shows that viscoelasticityhas a large influence on blade coating processes. In most cases, theprocess becomes Viscosities of Coating Compositions Viscosity testingtemperature: 100 F.

Material solids viscosity unstable when viscoelastic effects becomelarge. In this G CP set of experiments, the aqueous gelatin solutionsrangelatm 5 I0 1 u 15 140 ing from 5 to 20 percent SOlIdS worked verywell for the I8 350 sub-coat application. However, when the solidscontent 5 2% was increased above 2 0%, the viscoelastic property of u 3012900 the solution became more prominent, with the result f g 9 388 thatthe blade tended to lift off the paper and the coat- PVA 12 ing systembecame unstable. Therefore, it is felt that 20 356 the high solidsgelatin solutions do not lend themselves 2 3; i238 readily to blade coatapplications.

" 57 2,000 B. Methanol Solution of Polyvinyl Acetate (PVA) The 12percent solids PVA solution worked well as an undercoat material.However, the 20 percent solids PVA solution exhibited a pronouncedviscoelastic effect and caused some instability of the blade coat. Amethanol solution of polyvinyl acetate does not lend itself to use withan aqueous overcoat, because upon drying, the methanol escapes from thePVA and causes spots on the overcoats.

C. High-Solids Clay-Casein and High-Solids Baryta Both of these mixturesworked extremely well as subcoat fluids. The clay and baryta dispersionsare highly shear sensitive and thin quite readily in a blade coatinprocess. a

D. CMC

The CMC (3 to 8 percent solids) worked well as a sub-coat solution. Thismaterial exhibits the same shear thinning qualities as the high-solidsclay and baryta materials.

II. Overcoat (blade coat) Fluid Compositions A. Gelatin The solidscontent of the gelatin solutions used for the overcoat ranged from to 18percent. All mixtures that were used worked very well. In someinstances, a wetting agent was added to these mixtures to improve theirwetting qualities. Although other wetting agents probably could havebeen used satisfactorily, the one used was that sold under the trademarkTriton X-200 and having the chemical name P-(1,1, 33 tetramethylbutyl)phenoxyethoxyethyl sodium sulfonate.

B. PVA

The PVA mixtures (12 and 20 percent) solids were found to be goodovercoat fluids. There was some slight difficulty in coating this fluidcomposition because of its quick drying characteristics.

C. CMC

The 3 percent solids CMC was a very poor overcoat solution. The materialwould not distribute evenly as it was transferred from the blade to thecontinuous web and acted much like an elastic material. This problem wasfound to be characteristic of the CMC solution and not a limitation ofthe trailing blade slide coater process. The 8 percent solids CMC wasnot tried as an overcoat fluid due to the problems associated with thelower solids mixture.

Micro sections of various coated specimens made in the described trialswere made. It was found that subcoats of low solid mixtures were hard todetect on. the sections examined. The blade coater, when operating inthe correct manner, will apply a very small amount of solution toprecoated paper webs. For this reason, low-solids mixtures such as 3percent CMC andS percent gelatin are extremely hard to detect.High-solids mixtures such as the clay and baryta were more easily seen.It is evident from the sections examinedthatthe high-solids mixturesfill in the low areas of the continuous web and act as good, surfaceconditioners.

Micro sections of the three-layer coatings were examined to determine ifthe coating technique was capable of applying two overcoat layers thatwould remain separated after drying. The apparatus used in applying thethree layer coatings is shown in FIG. and the data of the test isdetailed in Table 3. The sections examined showed separation of thethree, layers fromone another.

As Tables 1, 2 and 3 show, the trailing blade slide coating process wastested at speeds rangingfrom 200 to 1,030 feet per minute. The upperlimit is not the coating process limit but the maximum speed at whichthe machine on which the tests were made could be safely run. Based onexperimental tests, it can be stated that the trailing blade slidecoating process is capable of speeds in excess of 1,000 feet perminute.

For the tests shown in Tables 1, 2 and 3, raw paper, precoated barytapaper and polyethylene coated paper were used as the continuous webmaterial, and all were coated successfully. A major draw back of thetrailing blade slide coater is its sensitivity to dust and paper fiberson the paper web which causes the coatings to be streaked. This problemwas more evident in the raw paper coatings because the paper and dustparticles were more numerous. As the blade contacts the paper web,foreign particles and paper fibers are contained by the blade edges.After a long period of coating, these particles build up and causestreaked finishes. Cleaning the raw paper web before coating didalleviate a large part of the problem and helped improve the coatingfinish. Using a precoated stock also helped to diminish streaks andimprove the finish. It is apparent from these experiments that if theweb being coated were a plastic film base, rather than paper, thisstreaking problem would have been less troublesome, particularly if thefilm surface was cleaned just ahead of the coating point. For a givenblade and hopper setting, the precoated paper stock allowed moresub-coat fluid to be applied to the web than a raw paper stock becausethe sub-coat cannot be forced into the stock by the trowelling action ofthe blade. Consequently, for a given setting of the blade, more of thesub-coat fluid is left on top of the web, because this situation causesmore hydrostatic pressure to be applied to the blade tip by the sub-coatfluid.

In the trials carried out, the coating and trailing blade angles had nomajor influences on the coating technique. Both angleswere variedthrough large ranges.

Although examples of certain synthetic polymeric solvent, solutions,aqueous pigment dispersions, and aqueous colloidal solutions which canbe coated by the present trailingblade slide coater technique assingleor distinct multiple layers have been disclosed, it will beobvious that there are many other fluid compositions which can besatisfactorily coated by this technique. For example, an aqueoussolution. of polyvinyl alcohol could be coated just as satisfactorily asthe mentioned solvent solution of polyvinyl acetate. This material couldbe satisfactorily used as a sub-coat under an overcoat of an aqueoussolution of gelatin, while the methanol solution. of polyvinyl acetatecould not because of the methanolcausingspots in the overcoat as itescapes during drying. Also, since aqueous gelatin solutions have been:shown to coat satisfactorily by the use of this technique it would bereasonable to assume that (photographic) gelatino silver halideemulsions and other coatings used in the photographic art could bereadily multiple coated by this technique, see US. Pat. No. 2,761,791,Russell, Sept. 4, 1956. It will also be apparent from the disclosure ofU.S. Pat. No. 2,761,791 that more than three layers of the same ordifferent fluid coating compositions could be simultaneously applied tothe surface of a web by this trailing blade slide coater technique bymerely combining more slide hoppers with the blade coater.

Although it is conceivable that the hoppers shown in FIGS. 2-5 could bemade from a single block of material, in order to facilitate thefabrication of the same and to make it possible to clean it out when itis desired to change from one coating composition to another, it is morepractical to make the hopper up from a number of separate sections, asshown, which can be readily assembled and disassembled. It should alsobe mentioned that if the coating compositions are of such a nature thatthey have to be heated or cooled while in the hopper in order to keepthem in a suitable condition for flow, then the hopper sections may beprovided with bores through which a heating or cooling liquid may becirculated, as is well known in the art.

This invention has been described with reference to particularembodiments thereof but it will be understood that variations andmodifications may be effected within the spirit and scope of theinvention.

I claim:

1. In a coating apparatus for applying two fluid coating compositions tothe surface of a web in superposed layer relation, the combination witha web guiding surface on which a web to be coated is adapted to becontinuously advanced while being held in a smooth condition, of ahopper spaced from said web guiding surface; a downwardly inclinedflexible trailing blade having one end fixed to said hopper and theother end engaging a first coating on the web on said guiding surfaceand deflected by such engagement to resiliently press against andtrowell the first coating onto the surface of said web as the web ismoved past said blade, said blade having an uppermost non-trowellingsurface and a lowermost trowelling surface; means for applying saidfirst fluid coating composition to the surface of said web in advance ofsaid blade to be trowelled by said trowelling surface of said blade intoa smooth layer; and means for continuously forming a layer of the otherfluid coating composition with the aid of gravity flow and flowing itonto the top of said trowelled first layer, and comprising a cavity insaid hopper for said other coating composition, and a discharge slotconnecting said cavity with the non-trowelling surface of said trailingblade at a point spaced above the flexed end thereof, and means formetering said other coating composition from said cavity through saiddischarge slot in the form of a layer and onto the non-trowellingsurface of said trailing blade, whereby the layer of coating compositionformed upon moving down the non-trowelling surface of said blade underthe influence of gravity flows off the deflected end thereof and ontothe top of the layer of first coating composition applied by thetrowelling action of said blade.

2. A coating apparatus as defined in claim 1, wherein said means forapplying a first fluid coating composition to the surface of said web inadvance of said blade comprises a second cavity in said hopper for saidfirst fluid coating composition, and a second discharge slot forconnecting said cavity with the trowelling surface of said trailingblade at a point spaced above the flexed end thereof; and means formetering said first fluid coating composition from said second cavitythrough said second discharge slot and onto the trowelling surface ofsaid blade in a layer which flows down the trowelling surface of theblade and onto said web surface.

3. A coating apparatus as defined in claim 1, wherein said means forapplying a first fluid coating composition to the surface of said webcomprises means for continuously depositing said first fluid coatingcomposition onto said web surface ahead of said trailing blade in aplurality of separate streams spaced transversely of said web and atsuch a rate as to form and maintain a puddle of fluid coatingcomposition behind and across the length of said blade.

4. A coating apparatus as defined in claim 1, wherein said means forapplying a first fluid coating composition to the surface of said webcomprises a reservoir for said coating composition, the bottom wall ofsaid reservoir formed by the surface of said web supported by saidguiding surface as it approaches said trailing blade, and one end wallof said reservoir formed by the free end of said trailing blade whichcontrols the deposition of the coating composition onto the surface ofsaid web as it passes said blade; and means for continuously feedingsaid first fluid composition into said reservoir at a rate at leastequal to that at which said composition is coated out onto said web.

5. A coating apparatus as defined in claim 1, characterized in that theend of said trailing blade that is trowelling said first coating istapered to a rounded point having a thickness less than half thethickness of the blade proper and wherein the upper surface of thetapered end joins the upper surface of the blade in a large radius sothat the layer of coating flowing down the blade is not subjected to asudden change of direction.

6. A coating apparatus as defined in claim 1 characterized by theinclusion of means for applying a third layer of fluid coatingcomposition along with said two layers mentioned, said means including asecond cavity in said hopper; a downwardly inclined slide surfaceintersecting said discharge slot at an acute angle, whereby a layer offluid coating composition flowing down said surface under the influenceof gravity is adapted to combine in surface relationship with the layerissuing from said discharge slot and flow down the surface of thetrailing blade in such strata relationship and flow off the end of saidblade onto the surface of the web in strata relationship with the layerof coating applied thereto by said blade; an elongated layer formingduct connecting said second cavity with said slide surface at a pointlocated above said discharge slot, and means for continuously feedingthe coating composition into said second cavity at a rate commensuratewith the thickness desired in the third layer after coat- 7. In acoating apparatus for applying a plurality of fluid coating compositionsonto the surface of a web in superposed distinct layer relationship, thecombination with a web guiding surface on which a web to be coated isadapted to be continuously moved while being held in a smooth condition;of a multiple coating hopper spaced from said web guiding surface; adownwardly inclined flexible trailing blade having a trowelling surfaceand a non-trowelling surface and further having one end fixed to saidhopper and the other end engaging a first coating on the exposed surfaceof the web on said guiding surface and deflected by such engagement topress against and trowel] the first coating onto the surface of said webas it is moved past said blade; means for applying said first fluidcoating composition to the surface of said web in advance of said bladeto be trowelled by said trowelling surface of said blade into a smoothlayer; and means for continuously forming each of the other fluidcoating compositions into individual layers, bringing them intosuperposed and distinct layer relationship with one another and thenwith the aid of gravity flow depositing them in superposed and distinctlayer relationship on the layer of the first fluid coating compositionapplied to the web; said last-mentioned means comprising a separatecavity in said hopper for each of said other coating compositions; aduct connecting the first of said cavities with the non-trowellingsurface of said trailing blade at a point spaced above the flexed endthereof; means for continuously metering one of said other coatingcompositions from said cavity and through said duct from which itemerges in the form of a layer which is deposited onto thenon-trowelling surface of said trailing blade; and means forcontinuously metering the remaining coating compositions from theirrespective cavities in the form of layers and combining said layers instrata relationship with each other and then with the layer of firstcoating composition as it is trowelled on the surface of said web.

8. In a coating apparatus for applying three fluid coating compositionsonto the surface of a web in superposed distinct layer relationship, thecombination with a web guiding surface on which a web to be coated isadapted to be moved while being held in a smooth condition; of amultiple coating hopper spaced from said web guiding surface; adownwardly inclined flexible trailing blade having a trowelling surfaceand a nontrowelling surface and further having one end fixed to saidhopper and the other end engaging a first coating on the exposed surfaceof the web on the guiding surface and deflected by such engagement topress against and trowel] the first coatingonto the surface of said webas it is moved past said blade; means for applying a first fluid coatingcomposition to the surface of said web in advance of said blade to betrowelled by said trowelling surface of said blade into a smooth layeron the web surface; and means for continuously forming each of the othertwo fluid coating compositions into individual layers, bringing theminto superposed and distinct layer relationship with one another andthen with the aid of gravity flow depositing them in superposed anddistinct relationship with the layer of the first coating compositionapplied to said web; said lastmentioned means comprising, a separatecavity in said hopper for each of the other two coating compositions; anelongated discharge slot communicating with one of said cavities anddirected downwardly onto the nontrowelling surface of said trailingblade at a point removed from the deflected end thereof; means forcontinuously metering one of the other two coating compositions from itscavity through said discharge slot in the form of a layer; a downwardlyinclined slide surface intersecting said discharge slot an an acuteangle; an elongated duct leading from the last cavity onto said slidesurface at a point above said discharge slot; and means for continuouslymetering said third coating composition from said last cavity throughsaid duct and onto said slide surface in the form of a layer which flowsdown said slide surface and onto the top of the layer issuing from saiddischarge slot and after which the two layers flow down said trailingblade and off the end thereof onto the layer of coating compositiontrowelled on the web by said trailing balde.

mg UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION P ten 3.756.195Dated September 4. 1973 lnuentofls) Joseph A. Mercier It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 45, after "apparatus" insert --for--;

lines 61-62, after "invention" insert --a--.

Column 5, line 5, after "said insert --puddle--;

line 6, delete "said puddle" and insert --web--.

Column 6, line 41, delete "0.008" and insert --'-0.08--. Column 6, TableI, line 67, an asterick should be inserted after "Gelatin;

- lines 68 and 69, a double asterisk should be inserted after "Gelatin".

Column 7, line 1, a double asterisk should be inserted after "Gelatin".

Signed and sealed this 9th day of April 1971 (SEAL) ,AIJIJCSJCI EDWARD II.FLETCHER,JH. I C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. In a coating apparatus for applying two fluid coating compositions tothe surface of a web in superposed layer relation, the combination witha web guiding surface on which a web to be coated is adapted to becontinuously advanced while being held in a smooth condition; of ahopper spaced from said web guiding surface; a downwardly inclinedflexible trailing blade having one end fixed to said hopper and theother end engaging a first coating on the web on said guiding surfaceand deflected by such engagement to resiliently press against andtrowell the first coating onto the surface of said web as the web ismoved past said blade, said blade having an uppermost nontrowellingsurface and a lowermost trowelling surface; means for applying saidfirst fluid coating composition to the surface of said web in advance ofsaid blade to be trowelled by said trowelling surface of said blade intoa smooth layer; and means for continuously forming a layer of the otherfluid coating composition with the aid of gravity flow and flowing itonto the top of said trowelled first layer, and comprising a cavity insaid hopper for said other coating composition; and a discharge slotconnecting said cavity with the non-trowelling surface of said trailingblade at a point spaced above the flexed end thereof; and means formetering said other coating composition from said cavity through saiddischarge slot in the form of a layer and onto the non-trowellingsurface of said trailing blade, whereby the layer of coating compositionformed upon moving down the non-trowelling surface of said blade underthe influence of gravity flows off the deflected end thereof and ontothe top of the layer of first coating composition applied by thetrowelling action of said blade.
 2. A coating apparatus as defined inclaim 1, wherein said means for applying a first fluid coatingcomposition to the surface of said web in advance of said bladecomprises a second cavity in said hopper for said first fluid coatingcomposition, and a second discharge slot for connecting said cavity withthe trowelling surface of said trailing blade at a point spaced abovethe flexed end thereof; and means for metering said first fluid coatingcomposition from said second cavity through said second discharge slotand onto the trowelling surface of said blade in a layer which flowsdown the trowelling surface of the blade and onto said web surface.
 3. Acoating apparatus as defineD in claim 1, wherein said means for applyinga first fluid coating composition to the surface of said web comprisesmeans for continuously depositing said first fluid coating compositiononto said web surface ahead of said trailing blade in a plurality ofseparate streams spaced transversely of said web and at such a rate asto form and maintain a puddle of fluid coating composition behind andacross the length of said blade.
 4. A coating apparatus as defined inclaim 1, wherein said means for applying a first fluid coatingcomposition to the surface of said web comprises a reservoir for saidcoating composition, the bottom wall of said reservoir formed by thesurface of said web supported by said guiding surface as it approachessaid trailing blade, and one end wall of said reservoir formed by thefree end of said trailing blade which controls the deposition of thecoating composition onto the surface of said web as it passes saidblade; and means for continuously feeding said first fluid compositioninto said reservoir at a rate at least equal to that at which saidcomposition is coated out onto said web.
 5. A coating apparatus asdefined in claim 1, characterized in that the end of said trailing bladethat is trowelling said first coating is tapered to a rounded pointhaving a thickness less than half the thickness of the blade proper andwherein the upper surface of the tapered end joins the upper surface ofthe blade in a large radius so that the layer of coating flowing downthe blade is not subjected to a sudden change of direction.
 6. A coatingapparatus as defined in claim 1 characterized by the inclusion of meansfor applying a third layer of fluid coating composition along with saidtwo layers mentioned, said means including a second cavity in saidhopper; a downwardly inclined slide surface intersecting said dischargeslot at an acute angle, whereby a layer of fluid coating compositionflowing down said surface under the influence of gravity is adapted tocombine in surface relationship with the layer issuing from saiddischarge slot and flow down the surface of the trailing blade in suchstrata relationship and flow off the end of said blade onto the surfaceof the web in strata relationship with the layer of coating appliedthereto by said blade; an elongated layer forming duct connecting saidsecond cavity with said slide surface at a point located above saiddischarge slot, and means for continuously feeding the coatingcomposition into said second cavity at a rate commensurate with thethickness desired in the third layer after coating.
 7. In a coatingapparatus for applying a plurality of fluid coating compositions ontothe surface of a web in superposed distinct layer relationship, thecombination with a web guiding surface on which a web to be coated isadapted to be continuously moved while being held in a smooth condition;of a multiple coating hopper spaced from said web guiding surface; adownwardly inclined flexible trailing blade having a trowelling surfaceand a non-trowelling surface and further having one end fixed to saidhopper and the other end engaging a first coating on the exposed surfaceof the web on said guiding surface and deflected by such engagement topress against and trowell the first coating onto the surface of said webas it is moved past said blade; means for applying said first fluidcoating composition to the surface of said web in advance of said bladeto be trowelled by said trowelling surface of said blade into a smoothlayer; and means for continuously forming each of the other fluidcoating compositions into individual layers, bringing them intosuperposed and distinct layer relationship with one another and thenwith the aid of gravity flow depositing them in superposed and distinctlayer relationship on the layer of the first fluid coating compositionapplied to the web; said last-mentioned means comprising a separatecavity in said hopper for each of said other coating compositions; aduct connectinG the first of said cavities with the non-trowellingsurface of said trailing blade at a point spaced above the flexed endthereof; means for continuously metering one of said other coatingcompositions from said cavity and through said duct from which itemerges in the form of a layer which is deposited onto thenon-trowelling surface of said trailing blade; and means forcontinuously metering the remaining coating compositions from theirrespective cavities in the form of layers and combining said layers instrata relationship with each other and then with the layer of firstcoating composition as it is trowelled on the surface of said web.
 8. Ina coating apparatus for applying three fluid coating compositions ontothe surface of a web in superposed distinct layer relationship, thecombination with a web guiding surface on which a web to be coated isadapted to be moved while being held in a smooth condition; of amultiple coating hopper spaced from said web guiding surface; adownwardly inclined flexible trailing blade having a trowelling surfaceand a non-trowelling surface and further having one end fixed to saidhopper and the other end engaging a first coating on the exposed surfaceof the web on the guiding surface and deflected by such engagement topress against and trowell the first coating onto the surface of said webas it is moved past said blade; means for applying a first fluid coatingcomposition to the surface of said web in advance of said blade to betrowelled by said trowelling surface of said blade into a smooth layeron the web surface; and means for continuously forming each of the othertwo fluid coating compositions into individual layers, bringing theminto superposed and distinct layer relationship with one another andthen with the aid of gravity flow depositing them in superposed anddistinct relationship with the layer of the first coating compositionapplied to said web; said last-mentioned means comprising, a separatecavity in said hopper for each of the other two coating compositions; anelongated discharge slot communicating with one of said cavities anddirected downwardly onto the non-trowelling surface of said trailingblade at a point removed from the deflected end thereof; means forcontinuously metering one of the other two coating compositions from itscavity through said discharge slot in the form of a layer; a downwardlyinclined slide surface intersecting said discharge slot an an acuteangle; an elongated duct leading from the last cavity onto said slidesurface at a point above said discharge slot; and means for continuouslymetering said third coating composition from said last cavity throughsaid duct and onto said slide surface in the form of a layer which flowsdown said slide surface and onto the top of the layer issuing from saiddischarge slot and after which the two layers flow down said trailingblade and off the end thereof onto the layer of coating compositiontrowelled on the web by said trailing balde.